It is very difficult to get effective rust inhibition in finished oils comprising highly paraffinic lubricating base oils. Highly paraffinic lubricating base oils include API Group II base oils having greater than 65% paraffinic chain carbons by ASTM D 3238, API Group III base oils having greater than 65% paraffinic chain carbons by ASTM D 3238, API Group IV base oils, polyinternal olefins, hydroisomerized Fischer-Tropsch wax, and Fischer-Tropsch oligomerized olefins. Others have approached this problem by using synergistic mixtures of different additives, and base oil blends to reduce the amount of highly paraffinic base oil in the finished oil. However, the current approaches have still not provided consistent passes in the 4 hour TORT B rust test using synthetic seawater, by ASTM D 665-02. The problem is notably more acute with higher viscosity oils, of ISO 100 grade or higher.
Others have made lubricant compositions with good rust inhibition, but these earlier compositions either had a different rust inhibitor formulation and/or they were made using different base oils than in the preferred embodiments of this invention. For example, U.S. Pat. No. 4,655,946 discloses a turbine engine oil that is resistant to seawater corrosion comprising a specific additive mixture different than what is disclosed in this invention, and preferably comprising a synthetic ester base oil. U.S. Pat. No. 4,701,273 describes lubricant compositions with good metal deactivation comprising antioxidants, amine phosphates and a preferred benzotriazole derivative.
There are a number of patents describing dual phosphorus and sulfur additives combined with amine phosphates for making superior load-carrying lubricants. These patents include U.S. Pat. No. 5,801,130; U.S. Pat. No. 5,789,358; U.S. Pat. No. 5,750,478; U.S. Pat. No. 5,679,627; U.S. Pat. No. 5,587,355; U.S. Pat. No. 5,585,029; and U.S. Pat. No. 5,582,760. None of these patents teach lubricating oils made with highly paraffinic base oils that have effective rust inhibition in seawater.
U.S. Pat. No. 6,180,575 teaches lubricating oils with anti-rust characteristics based on high quality base oils such as polyalphaolefins or hydroisomerized wax (petroleum or Fischer-Tropsch) with a secondary base oil, preferably a long chain alkylated aromatic. A synergistic combination of additives is used which is different than those of this invention. Unlike this invention, the additive mixture does not comprise a mixture of phosphate amines. The lubricating oils in U.S. Pat. No. 6,180,575 contain solubility improvers at levels much higher than are needed with preferred embodiments of our invention.
U.S. Pat. No. 5,104,558 teaches a rust-proofing oil composition for use in the surface treatment of steel sheets comprising at least one of a mineral oil and a synthetic oil as a base oil having a kinematic viscosity at 40° C. in the range of 5-50 cSt. The synthetic oil useful in U.S. Pat. No. 5,104,558 is selected from the group consisting of polybutene, alpha-olefin oligomer, alkylbenzene, alkylnaphthalene, diester, polyol ester, polyglycol, polyphenyl ether, tricresyl phosphate, silicone oil, perfluoroalkyl ether, normal paraffin and isoparaffin. Although this earlier patent included alkylnaphthalene and polyol ester as synthetic oils useful in the composition, there was no selection or understanding of the synthetic oil being potentially important as a solubility improver to improve rust inhibition. Alkylnaphthalene and polyol ester were grouped with other synthetic oils with high aniline points which are not the solubility improvers of this invention. U.S. Pat. No. 5,104,558 also used different rust inhibiting additives than those of this invention.